1. Field of the Invention
This invention relates to a multibeam scanning apparatus. More particularly, the present invention relates to a multibeam scanning apparatus adapted to optically scan the surface of a photosensitive drum by means of a plurality of light beams that are emitted from one or more than one light sources, reflected/deflected by a polygon mirror operating as light deflector and transmitted by way of an imaging optical system having an fxcex8 characteristic in order to record image information. A multibeam scanning apparatus according to the invention will find applications in the field of laser beam printers (LBPs) having a feature of carrying out a process of electrophotography and digital copying machines because it constantly secures regular intervals of scanning lines, which are essential for obtaining a high quality image.
2. Related Background Art
In conventional scanning optical apparatus to be used for image-forming apparatus such as laser beam printers and digital copying machines, the light beam emitted from a light source having a single light emitting section (light emitting spot) and optically modulated according to the image signal applied to it is collimated to a substantially parallel beam and made to pass through an aperture stop adapted to limit the light beam and then enter a cylindrical lens adapted to exert a predetermined refractive power on it only in the sub-scanning direction. While the substantially parallel light beam that enters the cylindrical lens then leaves the lens also as parallel light beam along the main-scanning plane, it is converged along the sub-scanning plane so that consequently it is focussed on the deflection plane (reflection plane) of a light deflector comprising a polygon mirror (rotary polygon mirror) to form a linear image there. The light beam reflected/deflected by the deflection plane of the polygon mirror is then led to the surface of a photosensitive drum by way of an imaging optical system (fxcex8 lens) having an fxcex8 characteristic so that the surface of the photosensitive drum may be optically scanned in order to record image information as the polygon mirror is driven to rotate.
In recent years, in response to the requirements for high speed and high resolution electrophotographic image forming apparatus, there have been proposed various multibeam scanning apparatus comprising a light source having a plurality of light emitting sections and adapted to lead so many light beams emitted from the light emitting sections and modulated optically and independently from each other to the surface of a photosensitive drum in order to optically and simultaneously scan the surface of the photosensitive drum with the plurality of light beams as so many scanning lines.
Meanwhile, the surface of a photosensitive drum operating as recording medium shows a reflectivity of several percent. Thus, in a light beam scanning apparatus, the light beam reflected/deflected by the polygon mirror is inclined by several degrees relative to the normal of the surface of the photosensitive drum in the sub-scanning direction before it strikes the surface of the photosensitive drum in order to eliminate the influence of the return light on the scanning operation of the light beam.
This is also true in multibeam scanning apparatus. More specifically, each and every one of the plurality of light beams reflected/deflected by the polygon mirror is inclined by several degrees relative to the normal of the surface of the photosensitive drum in the sub-scanning direction before it strikes the surface of the photosensitive drum. However, in the case of a multibeam scanning apparatus, the intervals separating the scanning lines can vary as a function of the angle of incidence of the light beams striking the surface of the photosensitive drum to give rise to irregular line intervals on the produced image.
Japanese Patent Application Laid-Open No. 10-243186 discloses a multibeam scanning apparatus adapted to shift the pixel density in the sub-scanning direction by utilizing the fact that the intervals separating the scanning lines can vary as a function of the angle of incidence of the light beams in the sub-scanning direction as the light beams strike the surface of the photosensitive drum. According to the patent document, the angle between the light beams and the surface of the photosensitive drum is modified by changing the position of an optical element of the imaging optical system, the light receiving position of the photosensitive drum and/or the profile of the photosensitive drum at the light receiving position.
However, the operation of changing the position of an optical element of the imaging optical system, the light receiving position of the photosensitive drum and/or the profile of the photosensitive drum at the light receiving position requires highly sophisticated techniques for fine adjustments and hence entails disadvantages from the precision point of view.
Therefore, it is the object of the present invention to provide a multibeam scanning apparatus that can accurately correct any irregularities of the intervals separating the scanning lines attributable to the angle of incidence of the light beams striking the photosensitive drum as viewed in the sub-scanning direction by selecting appropriate intervals for separating the plurality of light emitting sections of the light source means so as not to give rise to any irregular line intervals on the produced image.
According to the invention, the above object is achieved by providing a multibeam scanning apparatus comprising:
a light source having at least two or more than two light emitting sections arranged at intervals in the sub-scanning direction;
a light deflector for deflecting the two or more than two light beams emitted respectively from the two or more than two light emitting sections; and
a photosensitive drum to be scanned in the main-scanning direction by the two or more than two light beams deflected by the light deflector, the two or more than two light beams being adapted to strike the photosensitive drum with an angle inclined relative to the normal of the surface of the photosensitive drum as viewed in the sub-scanning direction;
wherein the intervals separating the light emitting sections of the light source in the sub-scanning direction are so selected as to arrange the scanning positions of the two or more than two light beams at regular intervals on the photosensitive drum.
According to the invention, there is also provided a multibeam scanning apparatus comprising:
a light source having a plurality of light emitting sections arranged at intervals in the sub-scanning direction;
a light deflector for deflecting the plurality of light beams emitted respectively from the plurality of light emitting sections; and
a photosensitive drum to be scanned in the main-scanning direction by the plurality of light beams deflected by the light deflector, the plurality of light beams being adapted to strike the photosensitive drum with an angle of inclination of xcex8 relative to the normal of the surface of the photosensitive drum as viewed in the sub-scanning direction so as to make the photosensitive drum to be scanned by the light beams at predetermined intervals;
wherein the intervals between any two adjacently located light beams of the plurality of light beams striking the photosensitive drum are so selected as to be smaller than the predetermined intervals on the basis of the angle xcex8.
According to the invention, there is also provided a multibeam scanning apparatus comprising:
a light source having a plurality of light emitting sections arranged at intervals in the sub-scanning direction;
a light deflector for deflecting the plurality of light beams emitted respectively from the plurality of light emitting sections; and
a photosensitive drum to be scanned in the main-scanning direction by the plurality of light beams deflected by the light deflector, the plurality of light beams being adapted to strike the photosensitive drum with an angle of inclination of xcex8 relative to the normal of the surface of the photosensitive drum as viewed in the sub-scanning direction so as to make the photosensitive drum to be scanned by the light beams at predetermined intervals;
wherein the intervals separating the light emitting sections of the light source in the sub-scanning direction are so selected as to be smaller than the predetermined intervals on the basis of the angle xcex8.
According to the invention, there is also provided a multibeam scanning apparatus comprising:
a light source having at least three or more than three light emitting sections arranged at intervals in the sub-scanning direction;
a light deflector for deflecting the three or more than three light beams emitted respectively from the three or more than three light emitting sections; and
a photosensitive drum to be scanned in the main-scanning direction by the three or more than three light beams deflected by the light deflector, the three or more than three light beams being adapted to strike the photosensitive drum with an angle inclined relative to the normal of the surface of the photosensitive drum as viewed in the sub-scanning direction;
wherein the intervals separating the light emitting sections of the light source in the sub-scanning direction are so selected as to make the difference between the angles of incidence of any two adjacently located light beams of the three or more than three light beams striking the photosensitive drum equal to a constant value.
According to the invention, there is also provided a multibeam scanning apparatus comprising:
a light source having a plurality of light emitting sections arranged at intervals in the sub-scanning direction, the light source being adapted to cyclically skip one or more than one scanning lines;
a light deflector for deflecting the plurality of light beams emitted from the light source; and
a photosensitive drum to be scanned in the main-scanning direction by the plurality of light beams deflected by the light deflector, the plurality of light beams being adapted to strike the photosensitive drum with an angle of inclination of xcex8 relative to the normal of the surface of the photosensitive drum as viewed in the sub-scanning direction so as to make the photosensitive drum to be scanned by the light beams at predetermined intervals;
wherein the intervals separating the light emitting sections of the light source in the sub-scanning direction are so selected as to be smaller than the predetermined intervals on the basis of the angle xcex8.
According to the invention, there is also provided a multibeam scanning apparatus comprising:
a light source having at least three or more than three light emitting sections arranged at intervals in the sub-scanning direction, the light source being adapted to cyclically skip one or more than one scanning lines;
a light deflector for deflecting the three or more than three light beams emitted from the light source; and
a photosensitive drum to be scanned in the main-scanning direction by the three or more than three light beams deflected by the light deflector, the three or more than three light beams being adapted to strike the photosensitive drum with an angle inclined relative to the normal of the surface of the photosensitive drum as viewed in the sub-scanning direction;
wherein the intervals separating the light emitting sections of the light source in the sub-scanning direction are so selected as to make the difference between the angles of incidence of any two adjacently located light beams of the three or more than three light beams striking the photosensitive drum equal to a constant value.
According to the invention, there is also provided a multibeam scanning apparatus comprising:
a light source having a plurality of light emitting sections arranged at intervals in the sub-scanning direction;
a light deflector for deflecting the plurality of light beams emitted respectively from the plurality of light emitting sections;
a photosensitive drum to be scanned in the main-scanning direction by the plurality of light beams deflected by the light deflector, the plurality of light beams being adapted to strike the photosensitive drum with an angle of inclination of e relative to the normal of the surface of the photosensitive drum as viewed in the sub-scanning direction; and
a detection element arranged at a position optically equivalent to that of the photosensitive drum to detect the imaging positions of the plurality of light beams emitted from the light source;
wherein the detection element is inclined in the sub-scanning direction by an angle of xcex8 relative to the normal of the surface of the photosensitive drum in order to adjust the intervals of the light emitting sections of the light source in the sub-scanning direction on the basis of the imaging positions as detected by the detection element.
According to the invention, there is also provided a multibeam scanning apparatus comprising:
a light source having at least three or more than three light emitting sections arranged at intervals in the sub-scanning direction, the light source being adapted to cyclically skip one or more than one light emitting sections and make a selected light emitting section emit a light beam;
a light deflector for deflecting the plurality of light beams emitted from the light source;
a photosensitive drum to be scanned in the main-scanning direction by the plurality of light beams deflected by the light deflector, the plurality of light beams being adapted to strike the photosensitive drum with an angle of inclination of xcex8 relative to the normal of the surface of the photosensitive drum as viewed in the sub-scanning direction; and
a detection element arranged at a position optically equivalent to that of the photosensitive drum to detect the imaging positions of the plurality of light beams emitted from the light source;
wherein the detection element is inclined in the sub-scanning direction by an angle of xcex8 relative to the normal of the surface of the photosensitive drum in order to adjust the intervals of the light emitting sections of the light source in the sub-scanning direction on the basis of the imaging positions as detected by the detection element.